One of the most widely used biocompatible polymers for medical use is hyaluronic acid (HA). It is a naturally occurring polysaccharide belonging to the group of glycosaminoglycans (GAGs). Hyaluronic acid and the other GAGs are negatively charged heteropolysaccharide chains which have a capacity to absorb large amounts of water. Hyaluronic acid and products derived from hyaluronic acid are widely used in the biomedical and cosmetic fields, for instance during viscosurgery and as a dermal filler.
Water-absorbing gels, or hydrogels, are widely used in the biomedical field. They are generally prepared by chemical cross-linking of polymers to infinite networks. While native hyaluronic acid and certain cross-linked hyaluronic acid products absorb water until they are completely dissolved, cross-linked hyaluronic acid gels typically absorb a certain amount of water until they are saturated, i.e. they have a finite liquid retention capacity, or swelling degree.
Since hyaluronic acid is present with identical chemical structure except for its molecular mass in most living organisms, it gives a minimum of reactions and allows for advanced medical uses. Cross-linking and/or other modifications of the hyaluronic acid molecule is necessary to improve its duration in vivo. Furthermore, such modifications affect the liquid retention capacity of the hyaluronic acid molecule. As a consequence thereof, hyaluronic acid has been the subject of many modification attempts.